Mosiah delivers the proclamation for the people to gather. While they had been invited to hear the king speak to them, it is probable that the invitation was an additional invitation to a festival that was to have been celebrated in any case. What we know of this occasion is that all of the people come, and they bring firstlings of the flocks for sacrifice and burnt offerings. That additional piece of information suggests that this was a holy occasion made more important with the king’s announcement. It is doubtful that all would come with sacrifices if they had only recently been notified of the king’s speech, and the seating of a new king did not require sacrifices under the law of Moses.
The idea that this was more than just the occasion of seating a new king will come in following verses. It is also important to remember that this comes at the end of many tribulations, so there are thanks offerings to be given, and perhaps offerings of atonement occasioned by actions during the previous conflicts.